test.cr

class Connected; end
class Disconnected; end

class Connection(T);
  def initialize
    Connection(Disconnected).new
  end

  def connect
    puts "Connected!"
    Connection(Connected).new
  end

  def disconnect
    puts "Disconnected!"
    Connection(Disconnected).new
  end
end

# class ConnectionDisconnected < Connection(Disconnected)
#   def connect
#     Connection(Connected).new
#   end
# end
#
# class ConnectionConnected < Connection(Connected)
#   def disconnect
#     Connection(Disconnected).new
#   end
# end

x = Connection(Disconnected).new # Connection<Disconnected>
# x.disconnect       # compile-time error

x = x.connect      # Connection<Connected>
# x.connect          # compile-time error
x = x.disconnect   # Connection<Disconnected>